In the field of optical communication technologies, an optical amplifier is used to extend a transmission distance of signal light. An example of the optical amplifier may include a rare earth-added optical fiber amplifier using a rare earth-added optical fiber for an optical amplification medium.
An example of the rare earth-added optical fiber may include erbium-doped optical fiber (EDF) produced by adding (doping) erbium ions, which are one example of rare earth ion, in an optical fiber core. An optical amplifier using EDF for an optical amplification medium is called EDFA. EDFA is being widely used to collectively amplify, for example, WDM (Wavelength Division Multiplexing) light obtained by wavelength division-multiplexing of the light with a plurality of wavelengths.
As another example of the optical amplifier, a semiconductor optical amplifier (SOA), a Raman optical amplifier using a distributed Raman scattering effect and the like have been known.
In some cases, the optical amplifier may prepare, for example, a plurality of configurations having different gains supported depending on a transmission distance of signal light (referred to as an “amplifier menu”). However, it may not be said that preparing different amplifier menus for different transmission distances is desirable in terms of maintenance, management and costs.
Therefore, there is a need to reduce the number of amplifier menus as much as possible. For example, an optical amplifier configuration capable of constitutively changing (or switching) a range of gain supported by one optical amplifier is contemplated.
The followings are reference documents.    [Document 1] Japanese Laid-open Patent Publication No. 03-103838 and    [Document 2] Japanese Laid-open Patent Publication No. 05-218974.